16 FBAGMENTS OF SCIENCE. 



pounds " constitute the mechanical equivalent of heat. 

 Now, a body falling from a height of 772 feet, has, upon 

 striking the earth, a velocity of 223 feet a second; and 

 if this velocity were imparted to the body, by any other 

 means, the quantity of heat generated by the stoppage 

 of its motion would be^that stated above. ' Six times 

 that velocity, or 1,338 feet, would not be an inordinate 

 one fgr a cannon-ball as it quits the gun. Hence, a 

 cannon-ball moving with a velocity of 1,338 feet a sec- 

 ond, wo aid, by collision, generate an amount of heat 

 competent to raise its own weight of water 36 degrees 

 Fahrenheit in temperature. If composed of iron, and 

 if all the heat generated were concentrated in the ball 

 itself, its temperature would be raised about 360 de- 

 grees Fahrenheit; because one degree in the case of 

 water is equivalent to about ten degrees in the case of 

 iron. In artillery practice, the heat generated is usu- 

 ally concentrated upon the front of the bolt, and on the 

 portion of the target first struck. By this concentra- 

 tion the heat developed becomes sufficiently intense to 

 raise the dust of the metal to incandescence, a flash of 

 light often accompanying collision with the target. 



Let us now fix our attention for a moment on the 

 gunpowder which urges the cannon-ball. This is com- 

 posed of combustible matter, which if burnt in the open 

 air would yield a certain amount of heat. It will not 

 yield this amount if it perform the work of urging a 

 ball. The heat then generated by the gunpowder will 

 fall short of that produced in the open air, by an amount 

 equivalent to the vis viva of the ball; and this exact 

 amount is restored by the ball on its collision with the 

 target. In this perfect way are heat and mechanical 

 motion connected. 



Broadly enunciated, the principle of the conserva- 

 tion of force asserts, that the quantity of force in the 



